The invention relates to optical smoke detectors for use, e.g., in early-warning automatic fire alarm systems.
Smoke detectors are distinguished among automatic fire detectors; on account of their exemplary ability to detect fires at a stage sufficiently early for timely deployment of countermeasures. Two types of smoke detectors are particularly important, namely ionization smoke detectors and optical smoke detectors. For smoke detection, the former depend on detecting adsorption of smoke particles on atmospheric ions, and the latter on optical effects in aerosols, e.g., the extinction of a beam of light by smoke ("extinction detectors") or the scattering of light by smoke particles ("scattered-light smoke detectors"). Since extinction by smoke particles is relatively weak, a relatively long measurement distance is required for reliable smoke detection by extinction detectors. This does not apply to the more widely used scattered-light smoke detectors, in which the measurement distance may be so short as to permit the design of so-called "point detectors".
The present invention more particularly relates to scattered-light smoke detectors. As an important design precaution, a scattered-light radiation detector must be prevented from responding to radiation not due to scattering by smoke particles. For example, in order to prevent ambient radiation from reaching the radiation detector, such a detector is provided with a light-shielding enclosure surrounding the optical beam in the measurement chamber. Smoke-inlet openings in the enclosure permit entry of ambient air, while substantially preventing the admission of light.
It is a precondition for reliable operation of a scattered-light smoke detector that no spurious light fall on the detector as reflected from the surfaces which delimit the measurement volume, or from extraneous matter deposited thereon. Designs have been proposed for minimizing the influence of surface reflections. Mainly, such proposals concern the design of optical labyrinths which largely absorb incident light; see, e.g., Swiss Patent Document CH-A5-590527.
Since smoke inlet openings cannot be made arbitrarily small, it is impossible to prevent admission of dust, fibers, or insects into the smoke detector, which may cause malfunctioning. Irradiated extraneous matter acts as a source of spurious light, and, if such light reaches the radiation detector, an electrical output signal may be produced as if smoke were present. As a result, in the interest of preventing frequent false alarms, scattered-light smoke detectors require regular cleaning, which may entail considerable costs.
Another, particularly vexing cause of false alarms arises with condensation of humidity on the measurement chamber surfaces, caused by temperature changes, and typically being of limited duration. This also produces spurious reflections which may be sensed by the radiation detector. Even cleaned detectors can be subject to this effect, so that servicing of detectors is of no avail in such cases.
This problem was addressed by methods which monitor the increase in surface reflections and which produce a trouble signal when secondary light from surface deposits exceeds a threshold level. Such methods fail, however, when deposition over time produces a signal which is similar to that due to a developing fire.
In order to overcome this drawback, two radiation detectors were included in a known smoke detector (see Japanese Patent Document JP-UM-131052), having fields of view encompassing different parts of a beam which lie at different distances from the radiation source and in which the radiation intensity differs in the presence of smoke. A logic circuit is included to ensure that an alarm is triggered only if the detected radiation intensities are in proper ratio. Since the fields of view of the two receivers encompass different surface portions, different reflection properties at such portions are unavoidable, as may be due to different degrees of dust deposition.
For the elimination of the influence of surface reflections, German Patent Document DE-C3-2754139 proposes a scattered-light smoke detector (see FIG. 1 of the present Drawing) in which a radiation source 2 disposed in a cylindrical enclosure 1 emits a beam of radiation across the measurement chamber. At another location of the cylinder surface, and away from the optical beam 3, a first radiation detector 7 is disposed such that its field of view 13 intercepts the optical beam 3 approximately at its midpoint and encompasses at least a portion of the optical beam 3. A second radiation detector 8 is disposed in the vicinity of the radiation detector 7, with a field of view 14 not touching the optical beam 3 but passing outside its perimeter, and directed to the same surface area 15 which is encompassed by the field of view 13 of the first radiation detector 7. By means of an evaluation circuit including a difference-forming element for forming the difference between the signals of the two radiation detectors 7 and 8, it is possible, under certain circumstances, to eliminate the influence of the spurious radiation from the surface area 15. In practice, however, it is impossible to adjust the fields of view of the radiation detectors 7 and 8 with sufficient accuracy for exact matching of the surface portions covered and for matching of the reflections. As a result, the problem of false alarms remains unsolved.
The main drawback of the known methods for the prevention of false alarms due to contamination lies with unrealistic demands on the optical systems, radiation sources and radiation detectors. In the presence of extraneous matter on lenses and diaphragms, their assigned tasks are impossible to fulfill as radiation paths fail expectations.
In view of the state of the art as described above, the invention is aimed at providing a scattered-light smoke detector which is not subject to the limitations of known scattered-light detectors. More particularly, the invention is aimed at providing a scattered-light smoke detector in which scattered light due to extraneous deposits is unambiguously recognized as such, so that false alarms due to contamination are prevented.